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A tub of a side dish, either tomato salsa or baba ganoush (eggplant), was left in a plastic bag for several months. The plastic bag was not airtight, but was wrapped somewhat tightly around the tub. Although it was supposed to be stored refrigerated, it was not (it was stored at room temperature). It also was not supposed to be stored for such a long duration.

When the bag with the tub inside was eventually handled, it was noticeably hot.

What biological reaction was taking place that would generate that kind of heat, especially after months of storage?

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As outlined in my comment, I hypothesise it must have been the growth of exothermic bacteria. As the name suggests, these produce heat. An especially interesting phenomenon concerns the danger of fire erupting from moist hay, as the bacteria feed from it and dry it simultaneously! This is outlined here. A memorable quote from that article:

Dry hay (stored at 15 percent moisture or less) is safe for long-term storage. However, if the hay has become wet the quality has been permanently changed and the potential fire hazard from spontaneous combustion increased.

The same principle underlies the heating of compost, about which you can read in greater detail here.

Most astounding I found the post dealing with the possible contribution of exothermic gut bacteria to our thermoregulation.

As of now, I don't know about the basic mechanisms by which these bacteria generate heat, but the abstract of this article implies that heat absorbing bacterial growth is extraordinary.

I myself have forgotten everything about chemistry, but from the comments below I gathered that what happens is similar to the oxidations of sugar. You can read examples here, which demonstrate that these are exothermic.

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    $\begingroup$ The mechanism is that they are essentially "burning" the organic compounds for energy, just like your own cells do, it's just that they are doing so fast enough to create a lot of heat. $\endgroup$ – Bryan Krause Jun 15 '17 at 16:45
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    $\begingroup$ Why is it a tautology? I guess my point was that there isn't any special mechanism, it's just from metabolism. I suppose maybe what you mean is "what is the mechanism by which they can continue to generate so much heat without simply sterilizing themselves." $\endgroup$ – Bryan Krause Jun 15 '17 at 16:52
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    $\begingroup$ I meant burning in the sense that the end reactions are converting carbohydrates into CO2 and water. Although the path of reactions isn't the same (and much slower), the products and thermal output are the same as if you lit the pile of hay on fire. So yes, oxidation. $\endgroup$ – Bryan Krause Jun 15 '17 at 17:12
  • $\begingroup$ @BryanKrause ok that makes perfect sense! $\endgroup$ – Ludi Jun 15 '17 at 17:16

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